A study of the complex genetics of left-right (L-R) axis malformations will be undertaken with an emphasis on those genes that are associated with common phenotypes of L-R disorders, including situs inversus, heterotaxia and organ isomerism. L-R defects can result from either environmental or genetic causes and it is the aim of these investigations to determine the genes responsible for both normal and abnormal L-R axis formation through the study of patients with these disorders. Mutations in one such gene, ZIC3, have been shown to be responsible for several familial and sporadic cases of heterotaxia (Gebbia et al. 1997). Other genes, including LEFTY A (Kosaki R, et al., 1999) and ACVR2B (Kosaki K, et al. 1999) have been linked to human situs abnormalities as well. We anticipate that many additional genes important for L-R development will be identified in the search for genetic causes of laterality disorders.